Hundreds of firefighters and police swarmed Ground Zero Sunday, the site where the World Trade Center once stood, in the largest security exercise here since the September 11, 2001 attacks.

As part of an elaborate dress rehearsal for a possible future terror strike, rescue workers exploded simulated bombs in a commuter train tunnel linking Manhattan to neighboring New Jersey, burrowed beneath the Hudson River.

“Full-scale exercises like today’s give us an opportunity to practice how to integrate the vast response resources available in New York City and establish a command structure under the Citywide Incident Management System,” said Joseph Bruno, commissioner at the city’s Office of Emergency Management.

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As the tunnel filled with smoke, scores of firefighters wearing oxygen masks practiced maneuvers to rescue some 700 to 800 passengers — the number who might be expected to become stranded along the vital commuter line in the event of a real disaster.

In fact, there were about 150 volunteers playing the role of passengers aboard the train early Sunday, usually a time of light travel along the city’s commuter train line.